Method for reducing the influence of a dc current component in the load current of an asynchronous motor

ABSTRACT

The aim of the invention is to provide a method, with which the load current and thus the load moment of an asynchronous motor that is controlled via a phase-controlled two-phase thyristor power controller can be easily influenced so as to allow a smooth starting operation. According to a first embodiment of the invention, the ignition point (t Ign+1 ) is determined in the controlled phase (L 1  and L 2 ) in order to adapt the flow angles of the subsequent current half waves. According to a second embodiment, the ignition point (t Ign ) of the subsequent current half waves is brought forward in both controlled phases (L 1  and L 2 ).

[0001] This application is the national phase under 35 U.S.C. § 371 ofPCT International Application No. PCT/DE02/04091 which has anInternational filing date of Nov. 4, 2002, which designated the UnitedStates of America and which claims priority on German Patent Applicationnumber DE 101 56 216.0 filed Nov. 15, 2001, the entire contents of whichare hereby incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention is generally directed to a method forreducing the influence of a DC component in the load current of anasynchronous motor.

BACKGROUND OF THE INVENTION

[0003] Three-phase controllers use a principle of phase gating tocontrol the amount of energy supplied to an electrical load. This allowsthe starting currents and torques in asynchronous machines to bereduced. The phase gating angle is used as a measure of the amount ofenergy supplied. Current half-cycles of alternate polarity flow throughthe load which is connected to the output of the three-phase controllerwith there being a time interval during which no current flows and whichis determined by the phase gating—also referred to as the delay—betweeneach two successive current half-cycles.

[0004] Three-phase controllers are normally equipped with three pairs ofthyristors which are connected back-to-back in parallel. However, sincethe thyristors become the cost-determining factor as the rating of thethree-phase controller increases, three-phase controllers having onlytwo pairs of thyristors which are connected back-to-back in parallel arealso used. In these so-called two-phase three-phase controllers, theremaining third phase is in the form of a conductor which cannot beswitched. A drive such as this is known from DE 30 09 445 A1.

[0005] SUGARY OF THE INVENTION

[0006] An embodiment of the invention is based on an object ofspecifying at least one method by which the load current, and hence theload torque, of an asynchronous motor which is controlled via atwo-phase thyristor three-phase controller with phase gating can beinfluenced with little complexity, for soft starting purposes.

[0007] According to an embodiment of the invention, an object isachieved by a first method, and/or by a second method.

[0008] An embodiment of the invention is based on the knowledge that, byvirtue of their principle of operation, when phase gating takes place inthe range of 75°±10° in two-phase three-phase controllers, thesecontrollers have a tendency to produce asymmetric current half-cycles ofpositive and negative polarity. In consequence, the successive positiveand negative current half-cycles which alternate with one another eachhave a different time duration and amplitude. This leads to a DCcomponent in the load current which, for example in the case ofasynchronous machines, produces braking torques and thus makes it harderor completely impossible to start them softly.

[0009] Thus the phase-gated load current, which has current half-cyclesof alternate polarity and of a different duration and amplitude, andaccordingly DC components which produce braking torques, can beinfluenced such that the DC components are reduced without usingmeasurement devices to determine the current/time integrals which areenclosed by the current half-cycles, thus ensuring that all theeffective currents have a uniform rise and profile in order to achievethe desired soft starting.

[0010] According to a second method of another embodiment, only currentflow information and in general no control loop information is required.Thus, this method can be implemented with a simpler and possiblyrestricted control device in comparison to the first method.Furthermore, the turn-off times are not recorded, so that it is possibleto save software and/or hardware modules.

[0011] According to one advantageous development of the first method, itis also advantageously possible, inter alia, to save currenttransformers for determination of the current flow angles, provided thatthe respective turn-on and turn-off time can be determined on the basisof a voltage rise across the associated thyristor in the thyristorthree-phase controller.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The various aspects of the invention as well as advantageousrefinements according to the features of the dependent claims will beexplained in more detail in the following text with reference toexemplary embodiments which are illustrated schematically in thedrawing, in which:

[0013]FIG. 1 shows a diagram with three phases of a load current beforeand after the use of a first method for reducing the influence of a DCcomponent, and

[0014]FIG. 2 shows a diagram with three phases of a load current beforeand after the use of a second method for reducing the influence of a DCcomponent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015]FIG. 1 uses a diagram to show the time profile of a three-phasecurrent during starting of an asynchronous machine using a two-phasethree-phase controller, before and after the use of a first method forreducing the influence of a DC component. This illustration shows acurrent i₁ and i₂ in a first and a second controlled phase L₁ and L₂,respectively, as well as a current i₃ in a third, uncontrolled phase L₃,in each case having pronounced DC components.

[0016] Furthermore, the currents i₁ and i₂ have a respectivelyassociated current i_(1K) and i_(2K) corrected according to the method,and the current i₃ has an associated current i_(3K), which is influencedby the correction according to the method, all in phase. In order tocontrol the phases L₁ to L₃, the phase gating is supplied by a functionthat can be determined via a ramp, in particular a linear ramp.

[0017] The first method for reducing the influence of the DC componentprovides that, first of all, the current flow angle γ_(n−1) in a firstcurrent half-cycle S_(1L1) is detected in the first controlled phase L₁from its turn-on time t_(Ign−1), and its turn-off time tExt1. Thecurrent flow angle γ_(n) in a subsequent second current half-cycleS_(2L1) is then recorded in the first controlled phase L₁ from itsturn-on time t_(Ign) and its turn-off time t_(Ext). The respectiveturn-on and turn-off time is recorded on the basis of a voltage riseacross the associated thyristor in the thyristor three-phase controller,and conventional control means can be used for this purpose.

[0018] Following this, then, the difference between the current flowangle γ_(n−1) in the first current half-cycle S_(1L1) and the currentflow angle γ_(n) in the second current half-cycle S_(2L1) is recorded.Finally, the turn-on time t_(Ign+1) in a subsequent, corrected, thirdcurrent half-cycle S_(3L1) is determined in the first controlled phaseL₁ on the basis of the recorded difference, in the sense of matching thecurrent flow angles in the current-half cycles.

[0019] The turn-on time is determined continuously for subsequentcurrent half-cycles on the basis of the recorded difference between thecurrent flow angles in the respective preceding current half-cycles.This results, inter alia, in the turn-on time t_(Ign+2) for asubsequent, corrected, fourth current half-cycle S_(4L1).

[0020] Both the third current half-cycle S_(3L1) which follows thesecond current half-cycle S_(2L1), together with the associated turn-onand turn-off times t_(Ign+1) and t_(Ext+1), respectively, as well as thefourth current half-cycle S_(4L1) which follows the third currenthalf-cycle S_(3L1), together with the associated turn-on and turn-offtimes t_(Ign+2) and t_(Ext+2), respectively, for the first controlledphase L₁ are superimposed in FIG. 1, in order to illustrate the matchingaccording to the method, with the current flow angles in the firstcurrent half-cycle S_(1L1) and the second current half-cycle S_(2L1) inthe first controlled phase L₁.

[0021] In consequence, in a schematic comparison, the corrected turn-ontime t_(Ign+1) in the third current half-cycle S_(3L1) occurs, accordingto the method, later than the original turn-on time t_(Ign−1) in thefirst current half-cycle S_(1L1) and, according to the method, thecorrected turn-on time t_(Ign+2) in the fourth current half-cycleS_(4L1) occurs earlier than the original turn-on time t_(Ign) in thesecond current half-cycle S_(2L1).

[0022] At the same time as the application of the first method, which isrelated to a single phase, to the first controlled phase L₁, this methodcan also be used without any interactions for the second controlledphase L₂, that is to say with the first phase L₁ being controlledindependently of the second phase L₂.

[0023]FIG. 1 in this case shows the commutation process KV on the basisof the falling flank F_(AB) in a current half-cycle S_(1L2) in thesecond controlled phase L₂, and on the basis of a rising flank FAN inthe fourth current half-cycle S_(4L1) in the first controlled phase L₁.

[0024] Since the two controlled phases L₁ and L₂ have currenthalf-cycles S_(1L2) and S_(4L1) with matched respective current flowangles γ_(s) and γ_(n+2) as a result of the first method, the currentprofile of the third, uncontrolled, phase L₃ is also advantageouslyinfluenced so that this results in all of the effective currents havinga uniform rise and profile. In principle, reducing the DC components ina load current also has a positive effect on any inductive measurementdevice which may be used, since this makes it possible to preventsaturation and thus to take precautions against incorrect measurements.

[0025] In contrast to symmetrical polarity driving and turning-on,turn-on times are advanced or delayed continuously by the use of acorrection factor C so as to effectively provide compensation fordifferent current flow angles in positive and negative currenthalf-cycles. Symmetrical polarity means that the time period duringwhich no current flows—the delay or phase gating angle α—between twosuccessive current half-cycles of alternate polarity during a transitionfrom a positive current half-cycle to a negative current half-cycle isexactly of the same magnitude as that for a transition from a negativeto a positive current half-cycle.

[0026] Normally, symmetrical polarity driving and turning-on areassociated with symmetrical phase driving and turning-on. Symmetricalphase means that the time period during which no current flows is alsoof the same magnitude in the sets of thyristor valves—three pairs ofthyristors which are connected back-to-back in parallel—in the threephases L₁ to L₃. Depending on the motor and the load state, thecorrection factor C may have a value of 0.1 to 0.4, in particular 0.2.

[0027] In a corresponding manner, when a thyristor valve set is turnedoff in the second current half-cycle S_(2L1), the corrected turn-on timeγ_(n+1) in each subsequent third current half-cycle S_(3L1) isdetermined using the following equation: $\begin{matrix}{t_{{Ign} + 1} = {t_{Ext} + {\frac{\alpha_{C}}{360^{\circ}}*T}}} & (1)\end{matrix}$

[0028] In this case, t_(Ext) indicates the turn-off time in the currenthalf-cycle which precedes the third current half-cycle S_(3L1), Tindicates the period duration, and α_(c) indicates the corrected phasegating angle. With an alternating current waveform at a frequency of 50Hz, the period duration T is, for example, 20 ms. The corrected phasegating angle α_(c) is accordingly determined using the followingequation:

α_(c)=α_(T) ±C*Δγ  (2)

[0029] In this case, α_(T) indicates the averaged phase gating angle,which is determined using the following equation: $\begin{matrix}{\alpha_{T} = \frac{\alpha_{n} + \alpha_{n - 1}}{2}} & (3)\end{matrix}$

[0030] In this case, α_(n−1), indicates the phase gating angle in afirst current half-cycle S_(1L1), and α_(n) indicates the phase gatingangle in a second current half-cycle S_(2L1).

[0031] Furthermore, C indicates the constant correction factor, which isbetween 0.1 and 0.4, and is in particular 0.2, and Δγ indicates thedifference between successive current flow angles using the followingequation:

Δγ=γ_(n)−γ_(n−1)  (4)

[0032] Owing to the alternating polarity between successive currenthalf-cycles, Δγ always describes the difference between the duration ofa positive current half-cycle and a negative current half-cycle.

[0033] In this case, γ_(n−1) indicates the current flow angle in a firstcurrent half-cycle S_(1L1) in accordance with the following equation:$\begin{matrix}{\gamma_{n - 1} = {\frac{t_{{Ext} - 1} - t_{{Ign} - 1}}{T}*360^{\circ}}} & (5)\end{matrix}$

[0034] In this case, t_(Ext−1) indicates the turn-off time in thecurrent half-cycle S_(1L1) which precedes the second current half-cycleS_(2L1), and t_(Ign−1) indicates the turn-on time in the first currenthalf-cycle S_(1L1) which precedes the second current half-cycle S_(2L1).

[0035] Furthermore, γ_(n) indicates the current flow angle in asubsequent second current half-cycle S_(2L1), in accordance with thefollowing equation: $\begin{matrix}{\gamma_{n} = {\frac{t_{Ext} - t_{Ign}}{T}*360^{\circ}}} & (6)\end{matrix}$

[0036] In this case, t_(Ign) indicates the turn-on time in the secondcurrent half-cycle S_(2L1) which follows the first current half-cycleS_(1L1).

[0037] In order to make it possible to carry out the correction afterthe next turn-off time, the current flow angle γ_(n) can be stored, inthe sense of a further first current half-cycle, for continuousmatching, using the following equation:

γ_(n−1)=γ_(n)  (7)

[0038] Advantageously, only time information and in general no controlloop information is required for the first method, with the timeinformation being available in a conventional controller for three-phasecontrollers in any case, in order to calculate the turn-on times, sothat the first method can be implemented with little complexity.

[0039] An asynchronous motor which is operated in this manner developsfrom the start of its drive a torque which increases continuously as thephase gating decreases, so that this asynchronous motor is acceleratedto the respective rated rotation speed within a time period of about 2 sto 4 s, for soft starting purposes. Furthermore, in comparison toconventional drives with a two-phase three-phase controller, theeffective values of the currents i₁ to i₃ are reduced by reducing DCcomponents.

[0040] An idea of an embodiment of the present invention is to equalizethe duration of successive current half-cycles of alternate polarity inone and the same phase, in order to suppress DC components when thephase gating values are in the region of 7520 ±10°. The correction isgenerally maintained until the end of the ramp function.

[0041] An embodiment of the invention as explained above may besummarized as follows:

[0042] In order to make it possible to influence the load current, andaccordingly influence the load torque, with little complexity for anasynchronous motor which is controlled by phase gating via a two-phasethyristor three-phase controller, in order to provide soft starting, thefirst method provides for the turn-on time (t_(Ign+1)) in the controlledphases (L₁ and L₂) to be determined in order to match the current flowangles in the subsequent current half-cycles.

[0043]FIG. 2 uses a diagram to show the time profile in approximatelyone period of a three-phase current during starting of an asynchronousmachine using a two-phase three-phase controller, before and after theuse of a second method for reducing the influence of a DC component. Theillustration shows a current i₁ and i₂ in a first and a secondcontrolled phase L₁ and L₂, respectively, and a current i₃ in a third,uncontrolled, phase L₃ in each case with pronounced DC components.

[0044] Furthermore, the currents i₁ and i₂ respectively have anassociated current i_(1K) and i_(2K), corrected according to the method,and the current i₃ has a current i_(3K), which is influenced by thecorrection according to the method, all in phase. In order to controlthe phases L₁ to L₃, the phase gating is supplied by means of a functionwhich can be determined via a ramp, in particular via a linear ramp.

[0045] The second method for reducing the influence of the DC componentis based first of all on recording the turn-on time t_(Ign1) in a firstcurrent half-cycle S_(1L1) in a first controlled phase L₁. Then, thecurrent flow in the first current half-cycle S_(1L2) in a secondcontrolled phase L₂ is recorded at the turn-on time t_(Ign−1) of thefirst current half-cycle S_(1L1) in the first controlled phase L₁.Finally, the turn-on time t_(Ign) in the subsequent second currenthalf-cycle S_(2L2) is advanced in the second controlled phase L₂ on thebasis of the recorded current flow, to a turn-on time t_(Ign+1) which issubsequent to this, in a third current half-cycle S_(3L2).

[0046]FIG. 2 shows the third current half-cycle S_(3L2) superimposed incomparison to the second current half-cycle S_(2L2), in which case, inprinciple, the third current half-cycle S_(3L2) follows the secondcurrent half-cycle S_(2L2). The advancing process is carried outcontinuously by means of a definable correction factor C, withnon-compliance with the condition resulting in no correction during therespective ramp function of the phase gating. Depending on the motor andthe load state, the correction factor C may have a value from −9° to−15°, in particular −12°, so that the second method is accordingly nolonger phase symmetrical.

[0047] The detection of these so-called trigger times for the correctionof the phase gating values and turn-on times is carried out continuouslyduring the ramp function for phase gating, thus resulting in acorrection which is independent of time and which is generallymaintained until the end of the ramp function.

[0048] At the same time as the check of the condition as to whethercurrent is flowing in the second controlled phase L₂, the second method,which relates to two phases, can likewise check the condition as towhether current is flowing in the first controlled phase L₁, thusallowing them to in each case be driven as a function of one another.

[0049] The advancing of the corresponding turn-on time means that agreater commutation current occurs, so that this results in a greatercurrent rise so that the corresponding turn-off time accordingly occurslater so that, in the end, the associated current flow angles areincreased or decreased.

[0050] By way of example, when carrying out the second method, two inputsignals, two output signals and a total of four interrupt routines maybe used on the control side. The two input signals are currentzero-crossing signals. Further, the output signals are thyristor turn-onsignals for the two controlled phases L₁ and L₂. The interrupt routinesare in each case triggered by the negative flank—which occurs every 10ms at a main frequency of 50 Hz—of a current zero-crossing signal. Inthis case, that particular phase gating is loaded into a so-called timerregister, and the timer is started.

[0051] When or if the timer overflows, a change is made to the interruptroutine in order there to generate a turn-on pulse for the respectivephase by reloading and starting of the timer. According to the method,the signal level of the corresponding input signal to the firstcontrolled phase L₁ and second controlled phase L₂ is checked throughoutthe entire ramp function for phase gating to the start of the generationof the turn-on pulse. If current is flowing, a first and/or a secondflag is set.

[0052] The respective flag is checked before that particular phasegating value is loaded into the timer register. If one or both flags isor are set, the respective phase gating angle is changed by the constantcorrection factor C before being loaded. If one flag or no flag is set,the respective phase gating angle for the first controlled phase L₁and/or for the second controlled phase L₂ is loaded into the timerregister without being changed, and conventional control means can beused for this purpose.

[0053] An asynchronous motor which is operated in this way generates atorque which increases continuously as the phase gating decreases fromthe start of the drive, so that it is accelerated to the respectiverated speed within a time period of about 2 s to 4 s, for soft startingpurposes. Furthermore, in comparison to conventional drives with atwo-phase three-phase controller, the effective values of the currentsi₁ to i₃ are reduced by reducing the DC components.

[0054] An idea of an embodiment of the present invention is to advancethe turn-on times of successive current half-cycles of alternatepolarity in two controlled phases as a function of a current flowcondition, in order to suppress DC components for phase gating values inthe region of 75°±10°.

[0055] An embodiment of the invention as explained above can besummarized as follows:

[0056] In order to make it possible to influence the load current andaccordingly the load torque of an asynchronous motor, which iscontrolled via a two-phase thyristor three-phase controller with phasegating, with little complexity for soft starting purposes, a secondmethod provides for the turn-on time (tin) in the subsequent currenthalf-cycles to be advanced in two controlled phases (L₁ and L₂).

[0057] Exemplary embodiments being thus described, it will be obviousthat the same may be varied in many ways. Such variations are not to beregarded as a departure from the spirit and scope of the presentinvention, and all such modifications as would be obvious to one skilledin the art are intended to be included within the scope of the followingclaims.

1. A method for reducing the influence of a DC component in a loadcurrent of an asynchronous motor controlled via a two-phase thyristorthree-phase controller with phase gating, comprising: detecting, in afirst controlled phase, a current flow angle in a first currenthalf-cycle, from turn-on time and turn-off time recording, in the firstcontrolled phase, a current flow angle in a subsequent second currenthalf-cycle, turn-on time and turn-off time; recording, a differencebetween the current flow angle in the first current half-cycle and thecurrent flow angle in the second current half-cycle; and determining, inthe first controlled phase, turn-on time in a subsequent third currenthalf-cycle on the basis of the recorded difference, in the sense ofmatching the current flow angles in the first and second current-halfcycles.
 2. The method as claimed in claim 1, wherein the turn-on timedetermined continuously for a subsequent current half-cycle on the basisof the recorded difference between the current flow angles in therespective preceding current half-cycles.
 3. The method as claimed inclaim 1 wherein the turn-on time is determined using a definablecorrection factor.
 4. The method as claimed in claim 3, wherein thecorrection factor in the range of 0.1 to 0.4.
 5. The method as claimedin claim 1 wherein the respective turn-on and turn-off time is recordedon the basis of a voltage rise across the associated thyristor in thethyristor three-phase controller.
 6. The method as claimed in claim 1,wherein the turn-on time (t_(Ign+1)) in a respective subsequent thirdcurrent half-cycle (S_(3L1)) is determined using:$t_{{Ign} - 1} = {t_{Ext} + {\frac{\alpha_{C}}{360^{\circ}}*T}}$

where: t_(Ext)=the turn-off time in the current half-cycle whichprecedes the third current half-cycle, T=the period duration α_(c)=thecorrected phase gating angle where: α_(c)=α_(T) C*Δγ where α_(T)=theaveraged phase gating angle$\alpha_{T} = \frac{\alpha_{n} + \alpha_{n - 1}}{2}$

where: α_(n−1)=the phase gating angle in a first current half-cycleα_(n)=the phase gating angle in a second current half-cycle C=thecorrection factor (0.1 to 0.4, in particular 0.2) Δγ=the differencebetween successive current flow angles where: Δγ=γ_(n)−γ_(n−1) where:γ_(n−1)=the current flow angle in a first current half-cycle where:$\gamma_{n - 1} = {\frac{t_{{Ext} - 1} - t_{{Ign} - 1}}{T}*360^{\circ}}$

where: t_(Ext−1)=the turn-off time in the current half-cycle whichprecedes the second current half-cycle, t_(Ign−1)=the turn-on time inthe current half-cycle which precedes the second current half-cycle,where: γn=the current flow angle in a subsequent second currenthalf-cycle, where:$\gamma_{n} = {\frac{t_{Ext} - t_{Ign}}{T}*360{^\circ}}$

and the current flow angle in a further first current half-cycle forcontinuous matching where: γ_(n−1):=γ_(n) where: t_(Ign)=the turn-ontime in the current half-cycle which follows the first currenthalf-cycle.
 7. The method as claimed in claims 1, wherein the method isfor at least one of the first and the second phase in the respectivemutually independent drive.
 8. The method as claimed in claim 1, whereinthe phases and in-are driven in the form of ramps, for soft starting ofthe asynchronous motor.
 9. A method for reducing the influence of a DCcomponent in a load current of an asynchronous motor controlled via atwo-phase thyristor three-phase controller with phase gating,comprising: recording a turn-on time in a first current half-cycle in afirst controlled phase; recording the current flow in the first currenthalf-cycle in a second controlled phase, at the turn-on time of thefirst current half-cycle in the first controlled phase; and advancingthe turn-on time in the subsequent second current half-cycle in thesecond controlled phase on the basis of the recorded current flow. 10.The method as claimed in claim 9, wherein the turn-on time is determinedcontinuously for a subsequent current half-cycle using a definablecorrection factor.
 11. The method as claimed in claim 10, wherein thecorrection factor is in the range −9° to −15°.
 12. The method as claimedin claims 9, wherein the method is for at least one of the first andsecond phase in the respective mutually independent drive.
 13. Themethod as claimed in claim 9, wherein the phases are driven in the formof ramps, for soft starting of the asynchronous motor.
 14. The method asclaimed in claim 2, wherein the turn-on time is determined using adefinable correction factor.
 15. The method as claimed in claim 14,wherein the correction factor is in the range of 0.1 to 0.4.
 16. Themethod as claimed in claim 3, wherein the correction factor is 0.2. 17.The method as claimed in claim 14, wherein the correction factor is 0.2.18. The method as claimed in claim 2, wherein the respective turn-on andturn-off time is recorded on the basis of a voltage rise across theassociated thyristor in the thyristor three-phase controller.
 19. Themethod as claimed in claim 2, wherein the method is for at least one ofthe first and the second phase in the respective mutually independentdrive.
 20. The method as claimed in claim 2, wherein the phases aredriven in the form of ramps, for soft starting of the asynchronousmotor.
 21. A method for reducing the influence of a DC component in aload current of an asynchronous motor, comprising: detecting a currentflow angle in a first current half-cycle from turn-on time and turn-offtime; recording a current flow angle in a subsequent second currenthalf-cycle from turn-on time and turn-off time; recording a differencebetween the current flow angle in the first current half-cycle and thecurrent flow angle in the second current half-cycle; and determiningturn-on time in a subsequent third current half-cycle by matching thecurrent flow angles in the first and second current-half cycles usingthe recorded difference.
 22. The method as claimed in claim 21, whereinthe turn-on time is determined continuously for a subsequent currenthalf-cycle on the basis of the recorded difference between the currentflow angles in the respective preceding current half-cycles.
 23. Themethod as claimed in claim 21, wherein the turn-on time is determinedusing a definable correction factor.
 24. A method for reducing theinfluence of a DC component in a load current of an asynchronous motor,comprising: recording a turn-on time in a first current half-cycle in afirst controlled phase; recording the current flow in the first currenthalf-cycle in a second controlled phase, at the turn-on time of thefirst current half-cycle in the first controlled phase; and advancingthe turn-on time in the subsequent second current half-cycle in thesecond controlled phase on the basis of the recorded current flow. 25.The method as claimed in claim 24, wherein the turn-on time isdetermined continuously for a subsequent current half-cycle using adefinable correction factor.
 26. An apparatus for reducing the influenceof a DC component in a load current of an asynchronous motor,comprising: means for detecting a current flow angle in a first currenthalf-cycle from turn-on time and turn-off time; means for recording acurrent flow angle in a subsequent second current half-cycle fromturn-on time and turn-off time and for recording a difference betweenthe current flow angle in the first current half-cycle and the currentflow angle in the second current half-cycle; and means for determiningturn-on time in a subsequent third current half-cycle by matching thecurrent flow angles in the first and second current-half cycles usingthe recorded difference.
 27. The apparatus as claimed in claim 26,wherein the turn-on time is determined continuously for a subsequentcurrent half-cycle on the basis of the recorded difference between thecurrent flow angles in the respective preceding current half-cycles. 28.The apparatus as claimed in claim 26, wherein the turn-on time isdetermined using a definable correction factor.
 29. An apparatus forreducing the influence of a DC component in a load current of anasynchronous motor, comprising: means for recording a turn-on time in afirst current half-cycle in a first controlled phase and for recordingthe current flow in the first current half-cycle in a second controlledphase, at the turn-on time of the first current half-cycle in the firstcontrolled phase; and means for advancing the turn-on time in thesubsequent second current half-cycle in the second controlled phase onthe basis of the recorded current flow.
 30. The apparatus as claimed inclaim 29, wherein the turn-on time is determined continuously for asubsequent current half-cycle using a definable correction factor.